[biltong]

Make life easier for yourself and put all that code after your first case into another custom function.

pawn Код:

//at top of script
forward MyFunc();

//anywhere
public MyFunc()
{
    new MaxPlayers = GetMaxPlayers();
    for(new forplayerid; forplayerid < MaxPlayers; forplayerid++)
    {
         if(!IsPlayerConnected(forplayerid)) continue;
         new VehicleID = GetPlayerVehicleID(forplayerid), Float:Health;
         if(VehicleID)
         {
             GetVehicleHealth(VehicleID, Health);
             if(Health < 500) return SendClientMessage(playerid, 0xFF0005FF, "Helti su ti preko 50 posto.");

             if(GetPlayerMoney(playerid) < 100) return SendClientMessage(playerid, 0xFF0005FF, "Nemas dovoljno novaca!");
             GivePlayerMoney(playerid, -100);
             new vehicleid = GetPlayerVehicleID(playerid); //compiler may throw an "already defined" error
             SetVehicleHealth(vehicleid,1000.0);
             SendClientMessage(playerid,0x00FF0AFF,"Popravio si svoje vozilo i platio 100$.");
         }
    }
    return 1;
}

//then your case code:
if(dialogid == 5512 && response)
  {

    switch(listitem)
    {
      case 0: MyFunc();
      //any other cases here...
    }
    return 1;
}

Hope that helps.